In the formation of mineral fibers, such as glass fibers, it is conventional to pull the mineral fibers through the apertures in a bushing, which forms the bottom of a container for molten mineral material. The molten mineral material depends from these apertures in the form of inverted cones, and the mineral fibers are drawn from these cones. The bushings can have tips or can be tipless. Conventionally, the mineral fibers are pulled downwardly at high speeds by a winder positioned beneath the bushing. During production, the moving fibers pull a substantial flow of entrained air downwardly along with the fibers. The flow of entrained air is especially strong for large throughput bushings, such as a 4000-hole, 150 pounds-per-hour bushing. At the bushing level, horizontal flows of induced air travel toward the bushing where the flows merge and turn downwardly to form the entrained air flow. These flows of induced air act as a heat sink for the newly formed fibers and for the bushing itself.
A problem encountered in conventional fiber-forming operations is that when there is an interruption of production the flow of induced air ceases, and the loss of the heat sink function of the induced air causes a temporary increase in the average temperature of the bushing. The average bushing temperature is that which would result from averaging the temperatures measured at several locations on the bushing, including the bushing sidewalls. For purposes of control, however, a single control thermocouple is usually mounted on or near the bushing bottom wall, and is used to monitor the temperature of the bushing as a whole. The power supplied to the bushing is then reduced below operation levels because the temperature sensed by the control thermocouple is required to be maintained relatively stable, even during an interruption of production. The reduction in power to the bushing causes the average bushing temperature to drop during the production interruption. Upon the resumption of production, the induced air flows recommence rather rapidly. The bushing and its power supply, however, cannot react rapidly enough to the sudden reintroduction of the heat sink provided by the induced air flows. The result is that upon resumption of production there is an initial period of approximately four to eight minutes during which the average temperature of the bushing is below nominal operating conditions. As a consequence of the depressed average bushing temperature, the throughput is reduced and the fibers are too fine in diameter, resulting in an unacceptable product.
One attempt to solve this problem, which is known as cold-start yardage, has been to program the winder speed so that upon resumption of production the fact that the initial throughput is reduced is taken into account. This attempted solution has been only partially successful because the corrective requirements dictated by the physical characteristics of each individual fiber-forming position are not always satisfied by a generalized winder speed curve. Also, the generalized winder speed curve fails to take into account the duration of the interruption of production.
By employment of the present invention, the average temperature of the bushing, and the power required to maintain that temperature, are caused to remain substantially constant during an interruption of production and the resumption of production.